Method for optimising coverage by reshaping antenna pattern

ABSTRACT

The invention relates to a method of optimizing coverage by changing the antenna pattern ( 14 ). In a solution of the invention, the base station ( 10 ) orients the antenna lobe ( 14 ) in the vertical direction toward a terminal equipment ( 11-13 ) by changing the shape of the antenna lobe according to a timing advance. The nearer the terminal equipment ( 11-13 ) is to the base station ( 10 ), the more downward the antenna lobe ( 14 ) is oriented by turning the antenna lobe ( 14 ) primarily downward or by changing the shape of the antenna pattern ( 14 ). In this way, transmission power, and thereby interference, is reduced.

This application is the national phase of international applicationPCT/FI97/00317 filed May 26, 1997 which designated the U.S.

FIELD OF INVENTION

The invention relates to a method for optimising coverage, useful in aTDMA-based radio system which comprises several terminal equipments andbase stations, and in which a signal is sent in time slots, apropagation delay of the signal between the terminal equipment and thebase station is defined, and the antenna lobe of the base station isoriented.

BACKGROUND OF INVENTION

An example of a typical time divisional radio system is the GSM (GlobalSystem for Mobile communication). Multiple access is implemented byproviding several carrier waves, each carrier wave having at most eightsimultaneous users. In order that the signals of these eight users couldbe separated from one another, they are sent as bursts in predefinedtime slots. In other words, a transmitter transmits only at a predefinedtime instant. A frame comprises the time slots of eight users, andrepetition of the frame allows transmission of even a large quantity ofdata.

In prior art solutions, the coverage of a multiple access cellular radiosystem based on time division is optimised by adjusting the transmissionpower and/or orienting the antenna lobe in the horizontal direction. Theadjustment of the transmission power, however, is not always possible.In particular, when the receiver is located far or behind an obstacle,the transmitter has to use a high transmission power. The problem withhorizontal orientation of the antenna lobe is that the base station hasto know in what direction or location the terminal equipment is. Thispiece of information is not always available. Typically the base stationknows only the propagation delay of the signal between the base stationand the terminal equipment.

BRIEF DESCRIPTION OF INVENTION

The object of the present invention is to optimise the coverage area ofa base station in a situation where the base station does not know thelocation or direction of a terminal equipment. If, on the other hand,the base station knows the location or direction of the terminalequipment, the coverage area and the shape of the lobe can be furtheroptimised more effectively than in the prior art.

The object is achieved by a method as described in the introduction, themethod being characterized in that the antenna lobe of the base stationassigned to the terminal equipment is orientedsubscriber-terminal-specifically by changing the shape of the antennalobe at least in the vertical direction.

Major advantages are achieved by the method of the invention. Theorientation of the antenna lobe of the base station reduces the amountof transmission carried outside the cell, and so intercellularinterference is also reduced. When the antenna lobe is oriented toward aterminal equipment, the transmission power of the terminal equipment canalso be minimised, which reduces the overall interference level bothwithin a cell and in adjacent cells.

BRIEF DESCRIPTION OF FIGURES

In the following the invention will be described in greater detail withreference to the examples illustrated in the attached drawings, in which

FIG. 1 illustrates orientation of the antenna patter of a base station,

FIG. 2 illustrates orientation of the antenna pattern of a base station,

FIG. 3 illustrates orientation of the antenna pattern of a base station,

FIG. 4 illustrates orientation of the antenna pattern of a base stationto a diffraction point,

FIG. 5 illustrates orientation of the antenna pattern to a diffractionpoint,

FIG. 6 illustrates a GSM frame,

FIG. 7 illustrates a structure of a base station, and

FIG. 8 illustrates a Butler type antenna structure.

DESCRIPTION OF PREFERRED EMBODIMENTS

The solution provided by the invention can be used in digital andanalogue radio systems based on time division. The invention is heredescribed mainly with reference to the GSM cellular radio systemoperated by a digital TDMA method. The applicability of the invention,however, is not limited to the GSM system.

FIGS. 1 to 3 illustrate an inventive way of orienting an antenna,pattern in a cell of a cellular radio system. FIGS. 1 to 3 show a basestation 10 and terminal equipments 11-13. The base station 10 has anantenna lobe 14 that can be oriented. In addition to the major lobe, theantenna lobe 14 comprises minor lobes 15, which are irrelevant to theinvention. Hereinafter, the direction of the antenna lobe means theprimary direction of the major lobe.

In FIG. 1 the base station 10 communicates with terminal equipment 11,which is preferably a mobile station. The antenna lobe of the basestation 10 is oriented toward terminal equipment 11, which is hereassumed to be located near the edge of the actual coverage area of thecell, i.e. base station 10. The propagation delay of the signal betweenthe terminal equipment 11 and the base station 10 is then typically atits longest.

In FIG. 2 the base station 10 communicates with terminal equipment 12,which is closer to the base station 10 than terminal equipment 11. Thepropagation delay of the signal between the base station 10 and terminalequipment 12 is typically shorter than in the case of FIG. 1. Theantenna lobe 14 of the base station 10 is here tilted essentiallyvertically downward toward terminal equipment 12. The advantage achievedis that the transmission from the base station 10 is oriented primarilyto terminal equipment 12, and simultaneously, to a lesser degree than inthe prior art, to adjacent cells, in which the transmission from thebase station 10 causes interference. In addition, the transmission powerof the base station 10 can be held lower than in the prior art, sincethe transmission from the base station 10 is oriented particularly toterminal equipment 12. The transmission power of terminal equipment 12can also be held lower than in the prior art, since the antenna lobe 14of the base station 10 is also oriented toward terminal equipment 12during the listening mode.

FIG. 3 shows another embodiment of the invention. The antenna lobe 14 ofthe base station 10 can also be oriented by changing the shape of thelobe. This can be done, for example, simply by reducing or enlarging thewidth of the antenna lobe 14 in the vertical direction. The embodimentcan be applied together with or separately from the precedingembodiment. When terminal equipment 13 is close to the base station 10,the antenna lobe 14 of the base station 10 can be widened in thevertical direction, whereby the antenna lobe, i.e. major lobe 14, alsoextends to terminal equipments 13 nearby. The antenna lobe 14 radiatesfar at a low power, whereby little interference occurs in adjacentcells. When the base station 10 transmits to or listens to a distantstation, the antenna lobe 14 is narrowed. The situation is then the sameas in FIG. 1, where the narrow antenna lobe 14 is oriented towardterminal equipment 11.

FIG. 4 shows an otherwise similar situation as FIG. 2 except thatterminal equipment 12 is located in a valley or behind an obstacle sothat there is no direct line of sight from the antenna lobe 14 of thebase station 10 to terminal equipment 12. The antenna lobe 14 is thenoriented to a diffraction point 16. An edge of the obstacle cuts some ofthe radiation, but the edge is simultaneously a diffraction point 16 andfunctions as a source of a new wavefront in accordance with the Huygens'principle, the electromagnetic radiation that carries the signal beingdiffracted therefrom to the reception antenna of terminal equipment 12.Since the electromagnetic radiation travels in both directions in thesame way, a connection from terminal equipment 12 to the base station 10is also made possible by the diffraction.

FIG. 5 concerns shaping of the antenna lobe, i.e. antenna pattern 14. Ina preferred embodiment of the invention, the antenna lobe can be shapedessentially as desired. In this example, the antenna lobe 14 is shapedso that the primary direction of the lobe 14 is toward the diffractionpoint 16, from which part of the signal scatters to terminal equipment12. If, on the other hand, not enough scattering takes place, terminalequipment 12 may still hear the transmission as a reflection. Ascompared with FIGS. 1 to 4, the difference is that the antenna lobe 14is asymmetrical to the primary direction (main axis) of the antenna lobe14. Terminal equipments 11 and 13 do thus not hear, or hear badly, asignal destined to terminal equipment 12.

FIG. 6 shows a TDMA frame of the GSM system. The frame comprises eighttime slots 1 to 8, which stand for the instants when a certain terminalequipment or base station can transmit or receive signals. The signalsof the time slots in the frame use the same carrier wave, and thesignals are separated from one another by time division. In one timeslot is usually sent one burst that contains data, e.g. a normal burstthat comprises start and stop bits, data bits, and training sequencebits, 248 bits in all. A time slot can take up to 256.25 bits, includingthe protective periods, and the duration of a time slot is 3.692 μs,from which it follows that the duration of a frame is 576.92 μs.

In a preferred embodiment of the invention, the antenna lobe 14 of thebase station 10 is re-oriented in every time slot, preferably separatelyto each user, i.e. terminal equipment 11-13. Naturally, orientation ismost preferably performed by turning the antenna lobe toward a terminalequipment 11-13 or a diffraction point 16.

FIG. 7 is a block diagram of the essential parts of a typical structureof a base station 10. On the reception side A, the base station 10comprises an antenna 50, radio frequency means 51, an analogue/digitalconverter 52, and a demodulator 53, from which the signal propagates,where necessary, to a base station controller or the like (not shown inthe figure). On the transmission side B, the base station 10 comprises amodulator 56 receiving data from the base station controller, radiofrequency means 57, and an antenna 50. The operation of the antenna 50is preferably controlled in an inventive manner by an antenna controller60 on the basis of a control signal obtained from control means. Theantenna controller 60, in turn, is controlled by antenna control means59, which belong to control means 58 controlling the operation ofessentially all transmitter and receiver blocks. The radio frequencymeans 51 lower the radio frequency signal to an intermediate frequency,after which the A/D converter 52 converts the signal to digital form.The digital signal is, for example, demodulated in the demodulator 53.Correspondingly, the signal is, for example, modulated in the modulator56, after which the digital signal is converted to a radio frequencysignal by the radio frequency means 57. The radio frequency signal isoutput from the antenna 50. The base station processes the digitalsignal preferably by digital signal processing means, which aretypically microprocessor-based ASIC or VLSI circuits.

When signals are transmitted in the solution of the invention, thepropagation delay of the signal from the transmitter to the receiver ispreferably taken into account. Since the instant of reception must bethe same irrespective of the distance between the transmitter and thereceiver, a signal is transmitted earlier to a distant receiver than toa receiver nearby. Consequently, a time slot destined to a distantreceiver is transmitted with a suitable timing advance. The suitabletiming advance is defined as described in the prior art. In the solutionof the invention, the direction of the antenna lobe is changed as afunction of timing advance preferably so that the shorter the timingadvance, the more the antenna lobe is tilted downward. The downward tiltcan be achieved by changing the primary direction of the lobe, or byusing a larger vertical width, or by otherwise shaping the lobe so thatthe signal best reaches the terminal equipment. If there is no directline of sight from the base station to the terminal equipment, theprimary direction of the lobe is preferably toward a diffraction point,at which the signal is diffracted to a terminal equipment. Thepropagation delay of the signal from the transmitter to the receiver isdefined by control means 58, which also define the timing advance neededfor the signal transmitted.

In the solution provided by the invention, the transmitter and thereceiver may have a common antenna or separate antennas 50. The antenna50 is typically a group of antennas, which allows phasing of theantennas and thereby turning and/or shaping of the antenna lobe. Onepossible antenna group solution is a Butler type antenna of FIG. 8,which may be linear, planar (two-dimensional), or omnidirectional. Inthis antenna solution, the Butler matrix, which corresponds to theantenna controller 60 of FIG. 7, phases the signal obtained from theantenna elements. Control means 58 thus define the distance of theterminal equipment 11-13 (propagation delay of the signal), and antennacontrol means 59 define the phasing and amplitude of the antennas on thebasis of the information obtained from control means 58. The antennacontroller 60 controls the antenna 50 on the basis of a control signalobtained from antenna control means 59. The number of outputs of theantenna control means, i.e. in this case switch matrix 59, is the sameas the number of virtual antenna lobes 14. The number of antenna lobesneed not be the same, however, as the number of antennas 50. The phasingof a Butler type antenna group 50 is typically fixed, whereby a lobe 14with a desired direction or shape can be selected from a fixed number ofantenna lobes. Another potential antenna group solution is an adaptiveantenna, in which the antenna pattern is not fixed but can be changedand shaped according to the need.

The preferably vertical changing of the antenna lobe according to theinvention can be combined with horizontal changing of the antenna lobe,whereby the antenna lobe can be oriented even more accurately to aterminal equipment, and interference both within and outside the cellcan be reduced. The inventive method improves coverage both in ruralconditions and, in particular, in urban conditions.

Although the invention is described above with reference to the exampleillustrated in the attached drawings, it is to be understood that theinvention is not limited thereto but can be varied in many ways withinthe scope of the inventive idea presented in the attached claims.

What is claimed is:
 1. A method for optimizing coverage, useful in aTDMA-based radio system which includes several terminal equipments andbase stations, and in which a signal is sent in time slots, a signalpropagation delay between the terminal equipment and the base stationbeing defined, the method comprising: orienting an antenna lobe of thebase station assigned to the terminal equipment subscriber-terminalspecifically by changing a shape of the antenna lobe at least in avertical direction; and when there is no direct line of sight betweenthe terminal equipment and the base station because of an obstacle,orienting the antenna lobe to an edge of the obstacle between theterminal equipment and the base station so that part of the lobe isdiffracted to a terminal equipment.
 2. The method of claim 1, whereinthe antenna lobe is oriented toward the terminal equipment.
 3. Themethod of claim 1, wherein the antenna lobe is orientedsubscriber-terminal specifically according to the signal propagationdelay between the terminal equipment and the base station.
 4. The methodof claim 1, wherein the antenna lobe of the base station is re-orientedin every time slot.
 5. The method of claim 1, wherein the shorter thepropagation delay of the signal to the terminal equipment, the moredownward the antenna lobe of the base station is oriented.
 6. The methodof claim 1, wherein when an antenna group is used as an antenna at thebase station, the antenna lobe of the base station is oriented bychanging the mutual phasing of the antennas in the antenna group.
 7. Themethod of claim 1, wherein when a timing advance is defined for a timeslot based on the propagation delay of the signal between the terminalequipment and the base station, the shorter the timing advance, the moredownward the antenna lobe is oriented as a function of timing advance.8. The method of claim 1, wherein the antenna of the base station is anessentially horizontally omnidirectional antenna, which can be orientedat least in the vertical direction.